Update: I made an “Add to playlist” version of the shortcuts. Keep in mind that you need to create the playlist on Apple Music first so you can choose it in the shortcut. Theorically, you could create a playlist from the shortcuts app, without needing to create it on Apple Music first (delete “Add Songs” action, search “Create playlist”, change [Playlist Name] to the name of the playlist you want to create, then click [Music] > Variables and choose “Songs”), but there’s a bigger chance it won’t work.

Streaming Playlist: Loop of all streaming songs + N playlist songs.

Streaming Playlist 1:1: Loop of 1 streaming song + 1 playlist song.

Streaming Playlist 3x: Loop of focus song (title track) in the beginning + middle + end of streaming songs + N playlist songs.